1. Field of the Invention
The present invention relates, in general, to a method for the preparation of ester compound and, more particularly, use of enzyme to prepare ester compounds from vapor state substrates.
2. Description of the Prior Art
Ester compounds as used herein include ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, ethyl propionate, ethyl butyrate, which all are used as natural flavor.
Lipase, an esterase, catalyzes the production of ethyl acetate (Langrand et al., Biotechnol, Lett., 12, 581, 1990), as illustrated by the following reaction formula: EQU ethanol+acetic acid.fwdarw.ethyl acetate+water
To speak generally, this reaction is expressed as follows: EQU alcohol+carboxylic acid.fwdarw.ester compound+water
It is possible to synthesize ester compounds in chemically synthetic processes. However, as natural compounds are increasingly demanded, there have been undertaken extensive researches and studies for utilizing ferments of microorganism or enzymes, to produce ester compounds in a large quantity (Armstrong et al., Biotechnol. Bioeng. 26, 1038, 1984; Williams et al., Ann. New York Acad. Sci., 542, 406, 1988; Murray and Duff, Appl. Microbiol. Biotechnol. 33, 202, 1990; Fukuda et al., Curr. Genet., 20, 49, 1991; Fukuda et al., J. Ferm. Bioeng. 75, 288, 1993). The reactions suggested in supra articles are carried out in aqueous solution. It is reported that the reactivities of the enzymes are confirmed. However, those processes are problematic in that the solubility of ester compound produced in water is extremely low.
Enzymatic reaction systems (Langrand et al., Biotechnol. Lett. 12, 581, 1990; Welsh and Williams, Enzyme Microbiol. Technol., 12, 743, 1990; Carta et al., Biotechnol. Bioeng., 37, 1004, 1991; Manjon et al., Biotechnol. Lett., 13, 339, 1991) show mass transfer limitation in solvents other than water and it is difficult to isolate the products from the solvents (Dordick, Enzyme Microbiol. Technol., 11, 194, 1989; Kery et al., J. Chem. Tech. Biotechnol., 48, 201, 1990).
As explained above, it is recognized that solubility of ester compounds in aqueous solutions is low and recovery of them from other solvents (reaction media) is not easy. Taking advantage of vaporization of the substrates (reactants) and the ester compounds (products), the present inventor made an attempt to catalyze the reaction in the vapor state.
There had been an attempt to react ethanol vapor with acetic acid vapor in the presence of lipase. However, it is reported that, when ethanol vapor is treated with acetic acid vapor at 30.degree. C. in the presence of lipase from Candida rugosa, ethyl acetate is not produced (Ross and Schneider, Enzyme Microbiol. Technol.. 13, 370, 1991). It is believed that this is attributed to a fact that water content in the lipase is high.